Re: [Xen-devel] question about disk performance in domU

[xuehai zhang <hai@xxxxxxxxxxxxxxx>]

Renato and Tim,
Thank you for your replies.
Is there anyway to flush the dom0 file buffer before running hdparm on domU 
each time?
Xuehai

Santos, Jose Renato G wrote:
> > > -----Original Message-----
> > > From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx 
> > > [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> > > Tim Freeman
> > > Sent: Monday, November 21, 2005 12:45 PM
> > > To: xen-devel@xxxxxxxxxxxxxxxxxxx
> > > Cc: keahey@xxxxxxxxxxx; xuehai zhang
> > > Subject: Re: [Xen-devel] question about disk performance in domU
> > >
> > >
> > > So the "Timing buffered disk reads" show much higher 
> > > results.  I see the DMA zone is larger in domU, but since 
> > > this is mapped to a loopback file I'm guessing the physical 
> > > disk performance should only be affected by what dom0's DMA 
> > > zone is set to on the node running the domU if that is all 
> > > that was going on.  But dom0 performance seems comparable to 
> > > native linux. 
> > >
> > > Is this huge "timing buffered disk read" difference 
> > > accurate? Does the domU benefit from some other cache of the 
> > > loopback file in dom0?  
>  
>   Yes. It seems to me that this is the effect of the file buffer
>   cache on dom0. hdparm flushes the file cache on domU 
>   to make sure that there is no data in the file buffer cache 
>   when measuring device access times (reported as buffered disk
>   reads). However when  using VBDs mapped to files, the data is
>   also cached on dom0 file buffer. Therefore data is not coming
>   directly from the device but from dom0 file cache. Note how the 
>   amount of data that is read in 3 sec increases at each step,
>   since data read in previous steps come from dom0 file cache.
>  
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